Of the first generation H.sub.2 -receptor blockers useful in the treatment of peptic ulcer disease, the best known is cimetidine which presents the structural formula: ##STR3## Ranitidine ##STR4## is about 5 times as potent as cimetidine in blocking histamine induced gastric acid secretion in the dog and between 5 and 10 times as potent as cimetidine in suppressing pentagastrin induced gastric acid secretion.
Tiotidine ##STR5## is reportedly at least ten times as potent as cimetidine in both in vitro and in vivo studies of H.sub.2 receptor antagonism.
Etintidine ##STR6## differs from cimetidine in the N-propargyl substituent and is more potent than cimetidine as an H.sub.2 -receptor antagonist. Etintidine possesses some anticholinergic activity. Thus, both potency and selectivity of H.sub.2 -receptor antagonism activity varies with modification of the structure of cimetidine.